The linear induction motor (LIM) concept is over one hundred years old, but only in recent times has its use grown significantly, due mainly to improvements in solid state control technology. However, LIM efficiencies have remained very low and, generally, its potential to provide great thrust power has been limited by its inability to handle heat generated by maximum electrical currents which lead to thermal breakdown of the LIM winding insulation, and magnetic losses from large air gaps imposed by physical constraints of the LIM system.
Air bearing support and conveying systems available under the SAILRAIL trade-mark can uniquely overcome these particular LIM problems. As will be described hereinafter the magnetic air gap of the LIM can be minimized by relying on the air-film suspension feature of the SAILRAIL guided hydrostatic complaint bearing, and the compressed air supplies of the SAILRAIL rails can be ducted through the LIM windings to enable the utilization of very high currents and thereby realize very efficient thrust powers.
Air bearing support and conveying systems available under the SAILRAIL trade-mark generally involve a parallel pair of rails, each having a transversely concave upper surface, appropriate air ducts or ports running the length of the rails and small diameter nozzles extending through the body of the rail at an angle to the upper surface and to the longitudinal axis of the rail. The nozzles are connected to the air ducts so that pressurized air can be directed from the air ducts through the nozzles to the upper surface. A system of valves and sensors is arranged along the rails so that groups of the nozzles are pressurized only in a specific desired sequence.
A load to be carried in a SAILRAIL system may be supported by a standard pallet which in turn is supported on a rectangular deck to which is attached a number of complaint runners, there being at least one runner for each rail. When pressurized air is exiting from the rail nozzles any runner thereabove will be levitated above the rail, spaced therefrom by an air-film gap of a few millimetres in thickness, the levitation system acting as an air bearing. The air-film supported load can be moved along the rails with a gentle push by hand, by gravity, or by other suitable means. The angled nature of the nozzles reduces the entry losses to the rail/runner air gap and the air exiting the nozzles at an angle aids in the removal of dust or dirt particles that may have settled in the rail. Braking can be effected by shutting off the air flow to bring the runners into contact with the rails, although this braking method is preferred for emergencies rather than for normal operation.
Another mechanism for achieving forward movement and braking of the runners is the linear induction motor (LIM). It has already been suggested, as in U.S. Pat. Nos. 4,489,825; 4,616,960 and 4,887,706, that the rail could contain the primary of a linear induction motor and that the runner could contain the secondary of such a motor (or vice versa). It has also been suggested that the secondary could include one or more copper plates which conform generally to the curvature of the runner's lower surface.